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Often the length of message exceeds the block length. So, the block ciphers need some extension. Consider a block cipher of length $$n$$. We fix a key $$k$$, and denote the encryption function with this key as $$!E_k\colon \{0, 1\}^n \to \{0, 1\}^n$$
To encrypt a message $$m$$ that is longer than $$n$$, the message is decomposed into blocks of fixed size $$ r, m = m_1m_2…m_l$$. The individual blocks are encrypted iteratively.
The message block size $$r$$ need not equal $$n$$. In few modes of encryption, $$r$$ is smaller than $$n$$.
Also if the length of message $$m$$ is not an integral multiple of $$r$$, then we have to complete the last block. The last block of the message can be padded out with some bits and encrypted. After decryption, the receiver must remove the padding. Therefore he must know how many bits were padded. This can be achieved, for exmple, by storing the number of padded bits in the last byte of the last block.
[ This is a part of a series of post on Modes Of Encryption. I had to scribe a lecture as a requirement of a course on the Foundations Of Cryptology at the Indian Institute Of Technology. The scribe has been broken into smaller chunks so that it is easily readable. ]
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